The present invention relates to a soft undercarriage for a crawler vehicle.
As an undercarriage for a crawler vehicle there is known, for example, an undercarriage in which a sprocket, an idler and upper and lower track rollers are rotatably mounted to a track frame attached to a vehicle body, with an endless track being entrained about the sprocket, idler and upper and lower track rollers. In such known structure, however, an excessive impact force acts on the idler portion when the vehicle gets over an obstacle, and it is transmitted to the vehicle body, thus causing trouble or breakage of not only the idler but also the vehicle body and devices attached to the vehicle body. As a result, it becomes necessary for those components to be rigid and durable enough to withstand such impact force, and this leads to an increase in the weight and cost of the vehicle. Moreover, the vehicle becomes less comfortable for the operator to ride in because an excessive impact force is exerted on the vehicle body. Further, since the lower track rollers are directly attached to the track frame, a part of the endless track may not contact a road surface when the road surface presents complicated changes. As a result, the effective grounding area of the endless track becomes smaller and so the tractive force is reduced, thus allowing shoe slip (skidding of the endless track against the road surface) to occur easily and making an effective utilization of the engine output impossible.
Additionally, since there may be some lower track rollers not in contact with the road surface, the burden of the grounded lower track rollers becomes so much heavier and their wear is accelerated. If the size of the shafts and rollers of the lower track rollers is increased in an effort to solve this problem, an increase in the weight and cost of the vehicle results.
Moreover, since the lower track rollers are directly attached to the track frame, even a slight change of a road surface causes pitching and rolling of the entire vehicle body, thus leading to an increased fatigue of the operator. Besides, in the case of leveling of land by the use of a blade, a rough road causes an increased pitching of the vehicle body to the extent that the land leveling work is very difficult to perform.
Further, since the support shaft portion of the idler is slidably in planar contact with the wear plate portion of the track frame, the sliding portion permits an easy entry of earth and sand, so wears in an early stage and thus is poor in durability. Besides, as previously noted, the sliding portion of the idler is complicated in structure and requires a large number of parts, thus resulting in that the assembling performance is deteriorated and the number of assembling steps increases.
Moreover, the track frame is box-shaped in section and its torsional rigidity is poor in view of its structure, so if it is increased in size so as to have a rigidity sufficient to withstand torsional loads while retaining its structure, the cost of the vehicle is increased due to its increased weight, etc.
There also is known an undercarriage, as disclosed in U.S. Pat. Nos. 3,774,708 and 3,826,325 in which a first track frame and a second track frame are pivotally connected; a pair of front and rear idlers are attached to the first and second track frames; a sprocket is mounted to an upper part of the vehicle body; an endless track is entrained about the paired idlers and the sprocket; and lower track rollers are supported vertically pivotably by the first and second track frames. In this structure, however, since the sprocket is mounted in a high position, a power transmission mechanism, including final reduction gears, is disposed in a relatively high position of the vehicle body and the position of the center of gravity of the vehicle inevitably becomes higher, thus resulting in that the distribution of the pressure of ground contact on a slope becomes less uniform and the working efficiency is deteriorated.
Moreover, since a crank is connected to the first track frame pivotably through a pin and the rear idler and lower track rollers are pivotally connected to the crank, not only an increase in pitching and in the operator's fatigue results but also it becomes difficult to perform the land leveling work using a blade.
Further, when a ripper is forced into the ground, the vehicle body is lifted with the pivot center of a lower track roller positioned near the front idler as a fulcrum, so that the distance between the ripper and the fulcrum becomes shorter by the distance between the idler center and the pivot center in comparison with the undercarriage of the type first referred to, and the insertion force of the ripper is reduced. Besides, since the center of gravity is positioned high, both static and dynamic stabilities of the vehicle are deteriorated. Moreover, not only a longer endless track than conventional ones is required but also the vehicle becomes heavier because of the provision of an additional idler, which leads to an increased cost of the vehicle.